1. Faculty of Electrical Engineering and Informatics, Technical University, Letná str. 9, 04200 Košice, Slovakia
2. Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova str. 47, 04001 Košice, Slovakia
3. Institute of Physics, Faculty of Science, P.J. Safarik University, Park Angelium str.9, 04154 Košice, Slovakia
4. Prokhorov General Physics Institute, Russian Academy of Sciences, Vavilov str. 38, 119991 Moscow, Russia
5. Moscow Institute of Physics and Technology, Moscow Region, 141700 Russia
6. Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Krzhyzhanovsky str. 3, 03142 Kiev, Ukraine